This invention is concerned with manganese-activated zinc orthosilicate phosphor Zn.sub.2 SiO.sub.4 :Mn. The phosphor emits in the green region of the visible spectrum and is used, for example, in fluorescent lamps and cathode ray tubes. Zn.sub.2 SiO.sub.4 :Mn is disclosed in U.S. Pat. Nos. 2,109,984; 2,206,280; 2,210,087; 2,222,509; 2,241,939; 2,245,414; 2,247,142; 2,544,999; and 3,416,019.
One of the problems associated with Zn.sub.2 SiO.sub.4 :Mn is its relatively poor fluorescent lamp performance and maintenance. Poor maintenance means that the light output, or lumens per watt, of the phosphor decreases to a greater extent during lamp life than is desirable. This invention is concerned with manganese-activated zinc silicate phosphor having improved fluorescence efficiency.
The manufacturing of the phosphor material manganese-activated zinc orthosilicate (also referred to as willemite) for fluorescent lamp applications typically involves reaction among components formulated with a significant nonstoichiometric excess of silicon dioxide or silicic acid. Heretofore, excess silica (SiO.sub.2) has been deemed necessary to achieve efficient and complete, or nearly complete, incorporation of the high activator dopant concentration levels of the Mn.sup.++ ion typically used in the phosphor. See, for example, U.S. Pat. No. 2,245,414. Such synthesis procedures, however, are often plagued by incomplete blending, typically caused by the poor dispersion characteristics of the ZnO or ZnCO.sub.3 components. Known synthesis procedures also result in some manganese losses. Manganese losses are associated with high vapor pressure of some manganese intermediate compounds; incomplete or improper incorporation of the manganous ion (caused by the variety of oxidation states of manganese); plus other thermodynamic considerations. Existing synthetic procedures for this phosphor also result in variable particle size development.
In fluorescent lamps, willemite phosphors manufactured by these known methods exhibit a faint tan or yellow discoloration, relatively high lamp depreciation (or poor lamp life), and overall low quantum efficiency, which is a result of sensitivity to lamp manufacturing plus the lamp depreciation factor.
One method for achieving a white bodied manganese-activated zinc silicate phosphor without requiring excess silica is described in U.S. Pat. No. 3,416,019. However, this patent shows the addition of MgF.sub.2 to be essential for achieving a white bodied phosphor.
The importance of high-performance, green-emitting phosphors with low depreciation characteristics in fluorescent lamps has increased in recent years with the growing demand for high CRI performance lamps. For that reason, in particular, an improved manganese-activated zinc orthosilicate phosphor and an improved method for preparing manganese-activated zinc orthosilicate phosphor would represent a significant advancement in the art.